(a) Field of the Invention
The present invention relates to a power train of an automatic transmission. More particularly, the present invention relates to a power train of an automatic transmission that realizes nine forward speeds and two reverse speeds.
(b) Description of the Related Art
A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A power train of such an automatic transmission changes rotational speed and torque received from a torque converter of the automatic transmission, and accordingly transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed, and therefore a vehicle can have economical fuel mileage and better performance. For that reason, an automatic transmission that is able to realize more shift speeds is under continuous investigation.
In addition, with the same number of speeds, features of a power train, such as durability, efficiency in power transmission, and size, depend a lot on the layout of the combined planetary gear sets. Therefore, designs for a combining structure of a power train are also under continuous investigation.
A manual transmission that has too many shift-speeds causes inconvenience to a driver. Therefore, the advantageous features of having more shift-speeds are more important in an automatic transmission because an automatic transmission automatically controls the shifting operations.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.